Basically, everybody wanted to ride the gravy train and nobody wanted to be the killjoy that pulled the brakes before it flew off of the cliff. There was a lot of money to be made and nobody thought they would be the ones left holding the bag.
The movie The Big Short explained this pretty well.
I suggest reading the book as well.
It's a long timeline, but I'd say things started getting specific late 2006:
In December 2007 in Australia I saw a presentation on the coming collapse of the US subprime housing market - in a lunchtime seminar given by a (tech) colleague for interests sake.